Energetic Tug-of-War between Pt and Leaky TiO2: Positive and Negative Effects on the Function of Molecularly-Modified p-Si(111)|TiO2|Pt Photocathodes

Photoelectrochemical solar energy conversion to chemical products is a promising avenue towards renewable fuels, and TiO₂ deposition on semiconductor photocathodes is a widespread surface protection strategy. However, the electronic, thermodynamic, and kinetic effects of protection layers on interfacial energetics remains poorly understood. Here, electrochemical data (photocurrent onset and flatband potentials) of p-Si(111)|TiO₂|Pt is collected in contact with an outer-sphere redox couple. We find that onset potentials shift negatively as a function of increasing oxide thickness, but this effect is not reflected in the flatband potentials. This reveals the mechanism of these shifts is kinetic rather than thermodynamic,and suggests that hole leakage through the TiO₂ is responsible. When Pt nanoparticles are deposited onto the TiO₂, positive shifts in both the onset and flatband potentials are observed, revealing Pt catalysts are beneficial for more than just increased catalytic rates. These results highlight the importance of understanding the effects of surface modifications on the energetics at the interface in both catalytic and simple electron transfer systems.